i wrote a comment about this a while ago. reposted:

1. WW2 destruction and recovery. Around a third of the population was either dead or wounded. The most industrially and agriculturally developed areas (for one Ukraine) were devastated. After the war, USSR was economically and politically isolated, they did not benefit from the Marshall Plan like western europe. USSR invested into the eastern block to rebuild cities and industries there. A capitalist country under such economic and demographic strain would fold in 5 years imho. Later on, there were huge investments into the military industry, including support to socialist revolutions around the world, which put more strain on the economy.
2. Increasing economic complexity. After the war, the amount of commodities/materials/indicators in the economy has increased exponentially. As there was a labour shortage (see point 1.), the economic planning model had to be adjusted to accommodate the capacity of the Gosplan. There was a point where cybernetics was considered, but the idea was dropped for being quite experimental and costly to implement. So instead, a retreat to markets was implemented in the form of Kosygin reforms (profit was created as an indicator, other indicators were simplified etc.).
3. Kruschev and dilution of party goals. During his leadership, Kruschev implemented party reforms that have diluted its class character and down-played importance of struggle (like the anti-Stalin speech for instance). It was one of the issues that flung the party and USSR into a spiral of "reforms for the sake of reforms" rather than analysis them as either a retreat or advancement towards communism.
4. Sabotage and public property misuse (corruption). The limited market reforms + party ideological crisis has created a certain "proto-capitalist" class that mainly consisted of factory managers who often appropriated public property for personal gains. They were the main proponents (as a class) for market reforms, but they only really gain power after Gorbachev's liberalisation and legalisation of private enterprises. Before Gorbachev, this class was very marginal and could be dealt with anti-corruption measures and modernisation, policies that were proposed by Andropov in 1982.

to conclude, there were no deeply ingrained issues, but a lack of capacity, imposed by the war and global isolation.

I think we all know it was because Stalin ate all the grain with his comically large spoon.

Economic isolation and being forced to maintain a colossal army are probably the main reasons. Franco's reactionary Spain tried a policy of economic autarky and it didn't work any better for him. It also got hindered by entrenched bureaucratic elements that prevented the USSR from capitalizing on the potential strengths of having of planned economy, such as project OGAS getting shut down by government workers just because they didn't want to get displaced.

I think it became overly dogmatic in some aspects and too "rigid". The partnership in the Treaty of Friendship, Cooperation, and Mutual Assistance became more and more top-down instead of equal, which is in itself a contradiction of internationalist marxist policies that were advocated for. In response to NATO, the system became more of an extension to soviet bureaucracy, which angered many of the working class in the other pact states, because it felt like or "resembled" a system which felt too close to imperialist rulership with peripheral countries. Also, the COMECON failed to achieve economical convergence and rather deepened devolopmental gaps between the member states (for example compare Romania's economy to GDR and you will see what I mean). Some people also complain about the bureaucracy becoming it's own class but I think it's bs, I am not a leftcom. Although I think some were becoming too greedy in the way of wanting to own things a common worker could almost never own. This also stifled resentment.

So, to conclude I would say the Warsaw Pact did not fail because the idea of collective socialist defense was invalid. The threat of NATO was very real. It failed because its actual practice betrayed the very socialist and internationalist principles it claimed to uphold. It became a mechanism for bureaucratic control, masking deepening economic inequalities and national tensions. Its dissolution was the political-military consequence of the broader crisis of a specific, historically-formed model of socialism, one marked by bureaucratic deformation, economic stagnation, and a negation of genuine workers' democracy and national self-determination. Therefore, its end is seen not as a refutation of Marxism, but as the necessary collapse of a structure that had deviated from core Marxist principles.